Solar cell structure and composition and method for forming the same

ABSTRACT

A semiconductor structure including a bonding layer connecting a first semiconductor wafer layer to a second semiconductor wafer layer, the bonding layer including an electrically conductive carbonaceous component and a binder component.

PRIOR APPLICATION

This is a divisional application of U.S. patent application Ser. No.12/814,722 filed Jun. 14, 2010.

GOVERNMENT CONTRACT

This invention was made with Government support under NR0000-08-C-0159awarded by the National Reconnaissance Office. The Government of theUnited States may have certain rights in the invention.

FIELD

This application relates to wafer bonding, particularly in solar cells.More particularly, this application relates to compositions and methodsfor bonding solar subcells to form high-efficiency, multi junction solarcell structures.

BACKGROUND

Conventionally, high-efficiency III-V multi junction solar cells areformed by growing all of the component subcells lattice-matched to asingle substrate. Unfortunately, this approach limits material choicesand, hence, the band gaps available for solar cell design, therebyresulting in sub-optimal power conversion efficiencies.

In an alternative approach, each component subcell of a solar cellstructure is grown on the most suitable substrate. Then, the subcellsare integrated into a multi junction solar cell structure using waferbonding techniques.

One known wafer bonding technique employs transparent metal oxides as abonding agent. For example, indium tin oxide has been shown to haveacceptable optical transparency, as well as good electricalconductivity. However, indium tin oxide has presented difficulties inachieving high quality bonds over a large surface area.

Other known wafer bonding techniques include using thin metallicinterface layers or direct semiconductor-to-semiconductor bondingthrough heavily-doped, thick III-V interface layers.

Nonetheless, those skilled in the art continue to seek new wafer bondingtechniques, including wafer bonding techniques that may be used in theconstruction of high-efficiency solar cells.

SUMMARY

In one aspect, the disclosed semiconductor device structure may includea bonding layer connecting a first semiconductor wafer layer to a secondsemiconductor wafer layer, the bonding layer including an electricallyconductive carbonaceous component and a binder/adhesive component.

In another aspect, the disclosed solar cell structure may include atleast one top subcell connected to at least one bottom subcell by abonding layer, the bonding layer including carbon nanotubes and abinder/adhesive component.

In yet another aspect, disclosed is a method for assembling a solar cellstructure. The method may include the steps of (1) growing or depositingat least one bottom subcell on a bottom substrate, (2) growing ordepositing a least one top subcell on a top substrate, (3) applying abonding layer to the bottom subcell and/or the top subcell, the bondinglayer including an electrically conductive component and abinder/adhesive component, (4) connecting the top subcell to the bottomsubcell such that the bonding layer is disposed therebetween and (5) ifnecessary (e.g., for optical transparency or electrical conductivity)removing the top substrate from the top subcell(s).

Other aspects of the disclosed solar cell structure and composition andmethod for forming the same will become apparent from the followingdescription, the accompanying drawings and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic side elevational view of a solar cell structureassembled in accordance with one aspect of the disclosure;

FIG. 2A is a schematic side elevational view of a bottom subcellassembly used to form the solar cell structure of FIG. 1;

FIG. 2B is a schematic side elevational view of a top subcell assemblyused to assemble the solar cell structure of FIG. 1;

FIG. 3 is a schematic side elevational view of a solar cell pre-assemblyformed by bonding the top subcell assembly of FIG. 2B to the bottomsubcell assembly of FIG. 2A;

FIG. 4 is a schematic side elevational view of the solar cellpre-assembly of FIG. 3 shown with the second substrate removedtherefrom; and

FIG. 5 is a flow chart illustrating an aspect of the disclosed methodfor manufacturing a solar cell structure.

DETAILED DESCRIPTION

Referring to FIG. 1, one aspect of the disclosed solar cell structure,generally designated 10, may include at least one top subcell 12, abonding layer 14, at least one bottom subcell 16 and a bottom substrate18. The solar cell structure 10 may additionally include a bottomcontact layer 20, a top contact layer (e.g., contacts 22) and ananti-reflective coating layer 24. Other components may also be includedin the solar cell structure 10 without departing from the scope of thepresent disclosure.

The at least one top subcell 12 may include a layer (or multiple layers)of semiconductor material having a front surface 26 and a back surface28. The top contacts 22 and the anti-reflective coating layer 24 may bepositioned adjacent to the front surface 26 of the at least one topsubcell 12. The back surface 28 of the at least one top subcell 12 maybe adjacent to the bonding layer 14.

The at least one bottom subcell 16 may include a layer (or multiplelayers) of semiconductor material having a front surface 30 and a backsurface 32. The front surface 30 of the at least one bottom subcell 16may be adjacent to the bonding layer 14. The bottom substrate 18 may bepositioned adjacent to back surface 32 of the at least one bottomsubcell 16.

The bottom substrate 18 may include a front surface 34 and a backsurface 36. The at least one bottom subcell 16 may be positionedadjacent to the front surface 34 of the bottom substrate 18 and thebottom contact layer 20 may be positioned adjacent to the back surface36 of the bottom substrate 18.

The bonding layer 14 may include an electrically conductive componentand a binder component. The electrically conductive component of thebonding layer 14 may provide vertical electrical conductivity betweenthe at least one top subcell 12 and the at least one bottom subcell 16.The binder component of the bonding layer 14 may bond the at least onetop subcell 12 to the at least one bottom subcell 16.

The composition of the bonding layer 14 may be selected to providesufficient optical transparency, vertical electrical conductivity andsufficient bond strength. For example, for a five junction solar celldesign, the composition of the bonding layer 14 may be selected toprovide (1) optical transmission greater than 95 percent over theoptical wavelengths in the spectral range relevant to the at least onebottom subcell 16 (e.g., wavelength greater than 800 nanometers); (2)total electrical resistance of at most 1 Ohm-cm² (note: this totalresistance includes the contact resistance to the semiconductor layersadjacent to the bonding layer 14, as well as the vertical resistance ofthe bonding layer 14 itself); and (3) bond strength having sufficientrobustness to withstand the processing steps as well as the end-useoperating conditions (e.g., terrestrial or space conditions).

The electrically conductive component of the bonding layer 14 may beselected to provide the bonding layer 14 with the required electricalconductivity without substantially reducing the optical transparency ofthe bonding layer 14. Therefore, the amount of the electricallyconductive component in the bonding layer 14 may be dictated by thecomposition of the electrically conductive component.

In a first expression, the electrically conductive component of thebonding layer 14 may include an electrically conductive carbonaceousmaterial or a combination of electrically conductive carbonaceousmaterials. In a second expression, the electrically conductive componentof the bonding layer 14 may include a combination of electricallyconductive carbonaceous material and inorganic conductive material.

In a first implementation of the first expression, the electricallyconductive carbonaceous material may include carbon nanotubes. Forexample, the carbon nanotubes may be single-walled nanotubes having anaverage diameter of about 1 to 2 nanometers and a length of at least 1micron. The carbon nanotubes may be in bundles and may define void space(e.g., 50 percent void space) between the nanotubes that may receive thebinder component. Suitable carbon nanotube films infiltrated withbinders (described in greater detail below) are marketed under theINVISICON® brand by Eikos, Inc. of Franklin, Mass.

The binder component of the bonding layer 14 may be selected to providethe bonding layer 14 with robust bonding capability withoutsubstantially reducing the optical transparency of the bonding layer 14.Therefore, the amount of the electrically conductive component in thebonding layer 14 may be dictated by the composition of the bindercomponent.

In one particular expression, the binder component of the bonding layer14 may be or may include a metal oxide, a metal nitride, a polymer, aninorganic-organic hybrid or combinations thereof. Examples of suitablebinder components include Al₂O₃ (alumina), TiO₂ (titania), HfO₂(hafnia), SiO₂ (silica), Si₄N₃ (silicon nitride), ZnO (zinc oxide) andIn₂O₃/SnO₂ (indium tin oxide). Examples of other suitable bindercomponents include silicones, such as Dow-Corning 93-500, and polymers,such as PDMS (polydimethylsiloxane).

As noted above, the electrically conductive component of the bondinglayer 14 may include carbon nanotubes that define voids and that renderthe bonding layer 14 electrically conductive. The binder component ofthe bonding layer 14 may infiltrate the voids defined by theelectrically conductive component to form a film. The binder componentmay provide the film with robustness and optical tunability.

The bonding layer 14 may be formed using various techniques, as isdescribed below. The resulting thickness of the bonding layer 14 mayrange, for example, from about 10 nanometers to about 100 nanometers.

Also disclosed is a method for manufacturing a semiconductor structure,such as the solar cell structure 10 shown in FIG. 1. Referring to FIG.5, one aspect of the disclosed method, generally designated 100, maybegin with the formation of a bottom subcell assembly 38 (FIG. 2A) atblocks 102 and 104 and the formation of a top subcell assembly 40 (FIG.2B) at blocks 106 and 108.

Referring to block 102 in FIG. 5, formation of the bottom subcellassembly 38 (FIG. 2A) may begin by forming the at least one bottomsubcell 16 on the bottom substrate 18. For example, the at least onebottom subcell 16 may be formed by epitaxially growing the at least onebottom subcell 16 on the bottom substrate 18. The at least one bottomsubcell 16 may be grown upright and lattice-matched (or nearlylattice-matched) to the bottom substrate 18. For example, the bottomsubstrate 18 may be an InP substrate and the at least one bottom subcell16 may include a GaInPAs subcell 42 and a GaInAs subcell 44.

Referring to block 104 in FIG. 5, a first bonding layer 48 may beapplied to the front surface 30 of the at least one bottom subcell 16.Those skilled in the art will appreciate that various techniques may beused to apply the disclosed bonding composition to form the firstbonding layer 48. In one implementation, the bonding layer 48 may beformed by applying a bonding composition formulated as a blend thatincludes the electrically conductive component and the binder component.In another implementation, the bonding layer 48 may be formed by firstapplying the electrically conductive component and then, separately,applying the binder component.

As one specific example, the bonding layer 48 may be formed in twosteps: (1) forming a carbon nanotube film and (2) infiltrating thecarbon nanotube film with a binder. An optional curing step may also beused. The carbon nanotube film may be formed by printing orspray-coating a carbon nanotube ink onto the front surface 30 of the atleast one bottom subcell 16. The carbon nanotube ink may be prepared asa dispersion of purified carbon nanotubes in a carrier, such as anaqueous carrier (e.g., water). The printing or spray-coating step may beperformed at low ambient temperatures to minimize premature evaporationof the carrier.

Once the carbon nanotube ink has dried, the remaining carbon nanotubeson the front surface 30 may be infiltrated with the binder. For example,the binder may be prepared as a liquid solution, such as a solution orsol-gel of binder. As a specific example, the binder solution mayinclude SiO₂ dissolved in alcohol. Then, the carbon nanotube film may bedip coated using the binder solution to infiltrate the carbon nanotubefilm with the binder component, thereby forming a bonding layer 48 thatincludes an electrically conductive component and a binder component.

Referring to block 106 in FIG. 5, formation of the top subcell assembly40 (FIG. 2B) may begin by forming the at least one top subcell 12 on thetop substrate 50. For example, the at least one top subcell 12 may beformed by epitaxially growing the at least one top subcell 12 on the topsubstrate 50. The at least one top subcell 12 may be grown inverted andlattice-matched (or nearly lattice-matched) to the top substrate 50. Forexample, the top substrate 50 may be a GaAs substrate and the at leastone top subcell 12 may include a GaAs subcell 52, a AlGaInAs subcell 54and a GaInP subcell 56.

Referring to block 108 in FIG. 5, a second bonding layer 58 may beapplied to the back surface 28 of the at least one top subcell 12. Thesecond bonding layer 58 may be applied using various techniques, asdescribed above.

At this point, those skilled in the art will appreciate that bothsubcell assemblies 38, 40 do not need a bonding layer 48, 58. Rather, inan alternative aspect, only one of the bottom 38 and top 40 subcellassemblies may be provided with a bonding layer 48, 58.

Referring to block 110 in FIG. 5, the top subcell assembly 40 may beconnected to the bottom subcell assembly 38 by mating the first bondinglayer 48 with the second bonding layer 58 to form the bonding layer 14,as shown in FIG. 3. The bonding layer 14 may be cured as necessary. Atblock 112 (FIG. 5), the top substrate 50 may be removed if necessary(e.g., if not optically transparent or electrically conductive), asshown in FIG. 4, thereby exposing the front surface 26 of the at leastone top subcell 12. Finally, at block 114 (FIG. 5), the top contacts 22may be applied to the front surface 26 of the at least one top subcell12 and the bottom contact 20 may be applied to the back surface 36 ofthe bottom substrate 18, as shown in FIG. 1.

Accordingly, the disclosed bonding composition may be used to bond afirst semiconductor wafer to a second semiconductor wafer. Inparticular, the optical, electrical conductivity and bonding propertiesof the disclosed bonding composition facilitate use of the compositionto bond a top subcell stack to a bottom subcell stack to form a solarcell structure. As such, the top subcell stack may be grownlattice-matched or nearly lattice-matched (i.e., substantiallylattice-matched) to the top substrate and the bottom subcell stack maybe grown lattice-matched or nearly lattice-matched (i.e., substantiallylattice-matched) to the bottom (different type) substrate, and thedisclosed bonding composition may be used to physically and electricallyconnect the top subcell stack to the bottom subcell stack with minimaloptical degradation, particularly when carbon nanotubes are used as theelectrically conductive component of the bonding composition.

Although various aspects of the disclosed solar cell structure andcomposition and method for forming the same have been shown anddescribed, modifications may occur to those skilled in the art uponreading the specification. The present application includes suchmodifications and is limited only by the scope of the claims.

What is claimed is:
 1. A semiconductor structure comprising: a firstsemiconductor wafer layer including at least one top subcell, whereinsaid at least one top subcell includes one or more of the following: agallium arsenide (GaAs) subcell, an aluminum gallium indium arsenic(AlGaInAs) subcell, and a gallium indium phosphide (GaInP) subcell; asecond semiconductor wafer including a bottom substrate and at least onebottom subcell, wherein said at least one bottom subcell has beenepitaxially grown from said bottom substrate, and wherein said at leastone bottom subcell includes at least one of said following: galliumindium phosphide arsenic (GaInPAs) subcell and a gallium indium arsenic(GaInAs) subcell; and a bonding layer connecting said firstsemiconductor wafer layer to said second semiconductor wafer layer, saidbonding layer comprising an electrically conductive carbonaceouscomponent and a binder component that provides bonding capability toconnect said first semiconductor wafer layer to said secondsemiconductor wafer layer, said electrically conductive carbonaceouscomponent comprising single-walled carbon nanotubes having a length ofat least one micron, wherein said carbon nanotubes are in bundles thatdefine void spaces between said carbon nanotubes, said carbon nanotubesrendering said bonding layer electrically conductive and said bindercomponent of said bonding layer infiltrating said void spaces defined bysaid carbon nanotubes to form a film.
 2. The semiconductor structure ofclaim 1 wherein said bonding layer has an optical transmission of atleast 85 percent at a wavelength of at least 800 nanometers.
 3. Thesemiconductor structure of claim 1 wherein said carbon nanotubes have anaverage cross-sectional thickness in a range from about 1 to about 2nanometers.
 4. The semiconductor structure of claim 1 wherein saidbinder component comprises at least one of a metal oxide, a metalnitride, a semiconductor oxide, a semiconductor nitride, a polymer and asilicone.
 5. The semiconductor structure of claim 1 wherein said bindercomponent is selected from the group consisting of alumina, titania,hafnia, silica, silicon nitride, zinc oxide, indium tin oxide andcombinations thereof.
 6. The semiconductor structure of claim 1 whereinsaid bonding layer has a thickness in a range from about 10 nanometersto about 100 nanometers.
 7. The semiconductor structure of claim 1wherein said bonding layer has an optical transmission of at least 95percent at a wavelength of at least 800 nanometers.
 8. The semiconductorstructure of claim 1 wherein said second semiconductor wafer layerabsorbs light in a spectral range, and wherein said bonding layer has anoptical transmission of at least 85 percent throughout said spectralrange.
 9. The semiconductor structure of claim 1 wherein said bondinglayer has a total electrical resistance of at most 1 Ohm-cm².
 10. Asolar cell structure comprising: a first semiconductor wafer layerincluding a top substrate and at least one top subcell, wherein said atleast at least one top subcell has been epitaxially grown from said topsubstrate, and wherein said at least one top subcell includes one ormore of the following: a gallium arsenide (GaAs) subcell, an aluminumgallium indium arsenic (AlGaInAs) subcell, and a gallium indiumphosphide (GaInP) subcell; a second semiconductor wafer including abottom substrate and at least one bottom subcell, wherein said at leastat least one bottom subcell has been epitaxially grown from said bottomsubstrate, and wherein said at least one bottom subcell includes atleast one of said following: gallium indium phosphide arsenic (GaInPAs)subcell and a gallium indium arsenic (GaInAs) subcell; and a bondinglayer connecting said first semiconductor wafer layer to said secondsemiconductor wafer layer, said bonding layer comprising single-walledcarbon nanotubes having a length of at least one micron and a bindercomponent, wherein said bottom subcell operates in a spectral range, andwherein said bonding layer has an optical transmission of at least 85percent throughout said spectral range, and wherein said bindercomponent provides bonding capability to connect said firstsemiconductor wafer layer to said second semiconductor wafer layer, andthe carbon nanotubes are in bundles that define void spaces between saidcarbon nanotubes, said carbon nanotubes rendering said bonding layerelectrically conductive and said binder component of said bonding layerinfiltrating said void spaces defined by said carbon nanotubes to form afilm.
 11. The solar cell structure of claim 10 wherein said carbonnanotubes have an average cross-sectional thickness in a range fromabout 1 to about 2 nanometers.
 12. The solar cell structure of claim 10wherein said binder component comprises at least one of a metal oxide, ametal nitride, a semiconductor oxide, a semiconductor nitride, a polymerand a silicone.
 13. The solar cell structure of claim 10 wherein saidbinder component is selected from the group consisting of alumina,titania, hafnia, silica, silicon nitride, zinc oxide, indium tin oxideand combinations thereof.
 14. The solar cell structure of claim 10wherein said bonding layer has a thickness in a range from about 10nanometers to about 100 nanometers.
 15. The solar cell structure ofclaim 10 wherein said bonding layer has an optical transmission of atleast 95 percent at a wavelength of at least 800 nanometers.
 16. Thesolar cell structure of claim 10 wherein said bonding layer has a totalelectrical resistance of at most 1 Ohm-cm².
 17. The solar cell structureof claim 10 wherein the at least one top subcell includes one or morelayers of semiconductor material and has a front surface.
 18. The solarcell structure of claim 17 wherein an anti-reflective coating layer ispositioned adjacent to the front surface of the at least one topsubcell.
 19. The solar cell structure of claim 17 wherein one or morecontacts are positioned adjacent to the front surface of the at leastone top subcell.
 20. The semiconductor structure of claim 1 wherein saidfirst semiconductor wafer including a top substrate and said at least atleast one top subcell has been epitaxially grown from said topsubstrate.